Orthopedic replacement of damaged or degenerated natural wrist joints is well known in the orthopedic arts. Prior to the introduction of prosthetic joint replacement for the wrist, individuals suffering from a joint disease in the wrist such as radio-carpal arthritis were often surgically treated by a fusion procedure. Fusion involves repairing the injured wrist joint structures with a fixed plate or rod that stiffens the wrist. That is, the joint is fixed in position by a device that allows no movement of the wrist. While this was an improvement over a diseased or injured wrist joint it is clearly unsatisfactory.
Existing orthopedic prostheses for wrist joint implantation have a number of limitations. Currently, most prosthetic wrist implants provide the patient with only limited functionality of the wrist, otherwise the implant becomes unstable. The natural wrist is astonishingly flexible in its freedom of motion. If a prosthetic wrist implant does not provide sufficient motion in flexion, extension, radial deviation, or ulnar deviation, the patient may have difficulty performing many of the normal tasks of daily living. Ideally, after implantation of a wrist prosthesis, the wrist will have a range of motion equal to or at least approaching that of a natural wrist joint.
An important requirement for prosthetic wrist implants is to have an extremely secure attachment between the implant and the bones. Separation of the prosthetic wrist implant from the bones to which it has been secured can be a serious complication requiring a repeat surgical procedure to make repairs. Failure of the attachment between the wrist prosthesis and the bones to which it is attached will cause further damage to the bones in some circumstances, making it difficult or impossible to treat the wrist even with a replacement implant. Prosthetic wrist implants currently in use generally require resection of the peripheral rim of the distal radius with its important ligaments and soft tissue attachments. The loss of these ligamentous attachments tends to create instability (looseness) of the prosthetic wrist. To compensate for this instability, a more involved surgical procedure must be performed to make reattachments of the soft tissue to the bone.
In addition, some currently available prosthetic wrist implants require the resection of a substantial amount of bone from the carpal bone structures. This substantial resection relocates the normal wrist centers of rotation (or motion) and relocation of the centers of wrist rotation interferes with normal function of the wrist extensor and flexor tendons, alters tendon moment arms and, as a result, limits and weakens movement of the wrist in extension and flexion. Further, if a patient later needs another procedure at the same joint revision options are limited if excessive tissue has been resected.
Another shortcoming of existing prosthetic wrist implants is the limitation of torsional movement of the wrist related the elbow. The healthy hand and wrist are able to rotate about an axis generally parallel to that of the long axis of the forearm, both because of the rotation of the radius and the ulna about one another, and because of the rotation of the natural wrist bones with relation to the radius and the ulna. Currently available prosthetic wrist implants typically involve the secure attachment of a proximal component of the wrist implant to the distal end of the radius and a distal component to the carpus with fixed planes of motion that result in a loss of torsional range of motion. In addition, if the forces involved in torsional movement of the wrist are limited by the implant as with current designs, those forces are transferred to the bone-implant interfaces at the radius and the carpus, increasing the risk of the implant loosening, and contributing to implant failure.
Further, in some wrist implant designs, a single stem extends through the carpus and into one or more of the metacarpals. The distal component of these wrist implants tends to erode through the metacarpal bone and create instability of the carpal attachment. Consequently the distal component of the implant may loosen or fracture where the implant enters the bones of the hand. In addition, in the normal wrist there is some freedom of motion between the carpals and the metacarpals and a stem passing through the carpals and into the metacarpals limits that freedom of motion resulting in less than ideal function of the wrist after implantation.
In addition, it has been found that implants that allow metal to metal contact between the radial and carpal components tend to cause shedding of metal particles that may migrate into surrounding tissues and may cause tissue necrosis and consequent implant failure and other complications.
Thus, it would be valuable to provide an improved orthopedic wrist implant that would provide a range of motion simulating the natural wrist's range of motion as closely as possible. In addition, it would be desirable if a prosthetic wrist implant would provide an improved torsional range of motion and reduce the effect of torsional forces on the bone-implant interface. It would further be desirable that a prosthetic wrist implant provide a secure, strong, and stable attachment to the surrounding bones in order to provide a wrist implant that would have low complications related to implant loosening. Further, it would be beneficial to preserve the peripheral rim of distal radius as well as the sigmoid notch of the distal radius where it articulates with the head of the ulna. It would be preferable to avoid metal-to-metal contact between the radial and carpal components.